This invention relates generally to switched mode power supplies and is particularly directed to a protection circuit for a switched mode power supply subject to damage arising from line voltage transients.
Switched mode power supplies typically include the series arrangement of an inductance and bi-directional controllable switch means coupled to input terminals for receiving an input voltage, a tuning capacitor arranged in parallel with the inductance, a drive or control circuit providing switching pulses for driving the switch alternately into the conducting and the cut-off states, wherein the duration of the conducting state of the switch is a function of the output voltage, and a rectifier for providing a DC output voltage.
Switched mode power supplies are generally of the tuned or non-tuned type. In a tuned switched mode power supply, a substantially sinusoidal oscillation of large amplitude is provided across the inductance during the time interval in which the switch is cut off, with the frequency of oscillation determined by the value of the inductance and an associated capacitance. After completion of half a cycle of this oscillation, the switch conducts again causing the energy to be fed back to the input voltage. In a non-tuned switched mode power supply, the input voltage is kept substantially constant because the duration of conduction of the switch is controlled as a function of the output voltage. This is usually accomplished by means of pulse duration modulation of the drive pulses applied to the switch, with these pulses having a frequency which is either constant or which varies with changes in the output voltage. Operation of the switching transistor is frequently controlled by an oscillator, where the duty cycle of the oscillator is variable. A control circuit typically senses the output voltage across the load and compares it to an internal reference.
Start-up of these switched mode power supplies is frequently characterized by a "soft" start mode of operation. This allows the power supply to start from zero current and/or zero voltage over a stated period of time during start-up to prevent voltage overshoots and large peak currents. Excessive voltages and peak currents arise from the instantaneous mode of operation of the circuit components during power supply turn-on as well as from line voltage interruptions. These fluctuations occur within the various circuits of the power supply until it achieves a stabilized operating condition. While this transient period may exist only for durations measured in milliseconds, the existing transients may be of substantial size and put excessive stress upon various circuit components. In particular, components such as transistors, integrated circuits and diodes in the output portions of the power supply are highly susceptible to damage arising from fluctuations in power supply operation caused by line voltage interruptions.
Various approaches are taught in the prior art for protecting a system energized by a power supply from variations in its operation such as arising from line voltage transients. For example, U.S. Pat. No. 4,261,032 to Cavigelli discloses a high voltage supply for driving a cathode ray tube which includes a feedback coil which operates to render nonconducting a switching transistor when the current in the feedback circuit reaches a predetermined level corresponding to the saturation of the transistor switch. With the transistor switch thus rendered nonconductive, subsequent large currents therethrough which might damage the power supply as well as the cathode ray tube are prevented. U.S. Pat. No. 4,473,759 to Mahabadi discloses a power sensing circuit which responds to changes in applied circuit voltages such that subsequent circuitry is powered down when the applied supply voltage drops below a specified level and which also incorporates a predetermined delay for powering up the subsequent circuitry when the applied voltage rises back to the same specified level. U.S. Pat. No. 4,207,516 to Babcock discloses a switching regulator circuit for regulating an input voltage which includes a capacitor coupled to a control circuit and responsive to the input voltage for providing a bias signal to delay the switching signal in preventing regulator switch closure during the peak magnitude of the AC input voltage component for limiting the peak current in the regulator circuit. While providing protection for the power supply and systems energized thereby, these approaches are overly complicated and are not intended to provide power supply protection for line voltage interrupts of short duration. Still other approaches make use of series impedances such as a resistor or a thermistor or an inductor, but these solutions are either inefficient or expensive, or both.
The present invention is intended to provide a switched mode power supply protection capability from line voltage interruptions not heretofore available which makes use of the power supply's soft start mode of operation and components associated therewith to reactivate soft-start circuitry before re-application of a full line voltage to the power supply to prevent power supply output component damage arising from excessive currents therein.